The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An aircraft is propelled by several propulsion units each comprising a turbojet engine housed in a nacelle also accommodating a set of auxiliary actuating devices linked to its operation and providing various functions when the turbojet engine is in operation or at stop, such as for example, a thrust reverser system.
A known propulsion unit 1 is shown in FIG. 1 in isometric view and in a maintenance position.
The propulsion unit 1 comprises a nacelle 3 and a turbojet engine 5. The nacelle 1 generally has a tubular structure comprising an air inlet 7 upstream of the engine, a median section intended to surround a fan of the turbojet engine, and a downstream section intended to surround the combustion chamber of the turbojet engine and accommodating a thrust reverser device.
The thrust reverser device is, during landing of the aircraft, intended to improve the braking capacity thereof by redirecting forward at least part of the thrust generated by the turbojet engine. The thrust reverser device comprises in particular movable cowls 9 displaceable between, on the one hand, a deployed position in which they open in the nacelle a passage intended for the diverted flow, and on the other hand, a retraction position in which they close this passage.
The fan of the turbojet engine is essentially composed of a rotary shaft carrying a plurality of blades. At their radial end, the blades are circumferentially surrounded by a fan casing 11.
The nacelle 3 includes downstream of the air inlet 7 two fan half-cowls 13a, 13b surrounding the fan casing 11 of the turbojet engine 5. Each fan half-cowl 13a, 13b is substantially semicylindrical, and define in a closed position (not shown) an aerodynamic continuity with an outer wall 15 of the nacelle 3 air inlet 7. In this closing position, the fan half-cowls 13a, 13b are locked on a fixed structure or with one another by a set of locks, along a lower longitudinal locking line in the case of an under-wing powerplant.
As shown in FIG. 1, the fan half-cowls 13a, 13b can be radially opened by pivoting about an upper longitudinal hinge line located in the proximity of an axis of a mast 17 to which the nacelle 5 is fastened, in order to allow access to certain components of the fan casing 11 during maintenance operations. In certain types of engines, the set of elements to be maintained is located only at the high-pressure zone of the engine (“core” volume). The maintenance operations are then concentrated at this zone of the engine.
The fan half-cowls 13a, 13b are opened thanks to at least one actuator, for example a cylinder 19, and are held in the open position thanks to at least one rod 21, the actuator and the rod each having a first end generally fixed on the fan casing 11 of the turbojet engine 5, and a second end fixed on the half cowl. In a known manner, the actuators can be produced in the form of hydraulic actuators.
In such a propulsion unit of the prior art, it was found first of all that the presence of the fan half-cowls causes a decrease in the laminar profile of the outer wall of the nacelle in contact with the outer air flow, resulting in parasitic drags.
Then, the actuators and the rods that constitute the system for opening and holding the fan half-cowls in the open maintenance position have a large mass, which weighs down the nacelle and which also consequently reduces its aerodynamic performances.